The invention pertains to optical communication systems.
The optical fiber communication infrastructure has expanded rapidly to satisfy the demands of communication customers requiring inexpensive, high-bandwidth transmission of voice, data, video, and other data streams. Inexpensive, high-speed communication has permitted the rapid expansion of Internet communication in addition to satisfying more traditional communication demands such as telephony.
Optical-fiber media have very large available communication bandwidths, and several approaches have been used to take advantage of this bandwidth. In one example, optical carriers have been modulated at high data rates, with data rates of 2.5 Gbit/s, 10.0 Gbit/s, and higher either installed or demonstrated. Using higher data rates, multiple data signals can be combined on a single optical carrier by modulating the optical carrier at different times for each of the data signals to be combined. This method is known as time-division multiplexing (TDM). Data bits are recovered from the modulated carrier and the detected bits are assigned to appropriate data signals, thereby demultiplexing the data signals.
In another method, multiple data streams are used to modulate optical carriers having different carrier wavelengths. The modulated carriers are combined and transmitted on a single optical fiber. To recover the data streams, the different carrier wavelengths are separated and the modulation of each carrier wavelength is detected. This method is known as wavelength-division multiplexing (xe2x80x9cWDMxe2x80x9d).
While these methods permit the use of more of the available optical fiber bandwidth, they also exhibit several practical limitations. For example, high-data-rate TDM requires optical sources, detectors, and associated electronics that are capable of very high-bandwidth operation. Such components are typically expensive. WDM systems require frequency-stable and accurately tuned optical sources. Such sources and other associated components also can be expensive.
An additional method that takes advantage of the high bandwidths available with optical fibers is known as code-division multiple access (xe2x80x9cCDMAxe2x80x9d). In this method, individual data signals are encoded with corresponding encoders, and the coded signals are combined and delivered via optical fiber or other communication medium. The data signals are recovered by decoding the combined coded signal with decoders, each of which corresponds to one (or more) of the encoders used to code the data signals. In this method, the separation (demultiplexing) of different data signals depends on the codes used by the encoders and decoders. The number of data signals that can be combined depends on the number of available codes and acceptable levels of channel crosstalk. Available codes tend to produce unacceptable levels of crosstalk between the decoded signals, i.e., a decoded data signal corresponding to a selected data signal contains artifacts due to other data signals. Such crosstalk is particularly troublesome in systems in which many data signals are to be multiplexed. For these reasons, improved methods and apparatus are needed for code-based multiplexing.
Codes and code-based multiplexing methods and apparatus are provided that permit increased numbers of data signals to be multiplexed while maintaining channel cross-talk and other coding artifacts at acceptable levels.
Code-based communication systems comprise a coder that applies a code selected from a set of synchronous, substantially orthogonal codes to a data stream. In specific embodiments, the codes are selected from a set of orthogonal codes and these codes are applied to data signals as temporal codes. In a representative embodiment, the codes are temporal phase codes.
Methods of coding a data stream include selecting a time interval and dividing the time interval into two or more time chips. Each of the time chips is assigned a modulation value based on a code. The data stream is divided into portions corresponding to the time chips, and the modulation values assigned to the time chips are coded onto corresponding portions of the data stream. In representative examples, the code is a discrete phase code, or a temporal phase code, and is selected from a set of synchronous, substantially orthogonal codes. In a specific example, the code is selected from the set consisting of the phase codes: (4xcfx80/3, 2xcfx80/3, 0, 2xcfx80/3, 4xcfx80/3, 0, 0, 0, 0), (4xcfx80/3, 0, 4xcfx80/3, 0, 0, 2xcfx80/3, 2xcfx80/3, 0, 0), (0, 4xcfx80/3, 2xcfx80/3, 0, 2xcfx80/3, 4xcfx80/3, 0, 0, 0), (2xcfx80/3, 4xcfx80/3, 0, 0, 0, 0, 4xcfx80/3, 2xcfx80/3, 0), (0, 0, 0, 4xcfx80/3, 2xcfx80/3, 0, 2xcfx80/3, 4xcfx80/3, 0), (0, 4xcfx80/3, 4xcfx80/3, 2xcfx80/3, 4xcfx80/3, 2xcfx80/3, 4xcfx80/3, 4xcfx80/3, 0), and (4xcfx80/3, 0, 2xcfx80/3, 4xcfx80/3, 4xcfx80/3, 4xcfx80/3, 4xcfx80/3, 2xcfx80/3, 0).
Coders for encoding an optical-data stream include a circulator that receives the optical-data stream; and an optical fiber that receives the optical-data stream from the circulator. The optical fiber includes two or more reflectors situated and configured to apply a code to the data stream. The code can be a synchronous, substantially orthogonal code selected from a set of substantially orthogonal codes.
Coders for coding an input-data stream include two or more reflectors situated and configured to selectively reflect portions of the input-data stream according to a synchronous code. According to representative embodiments, the synchronous code is an orthogonal code, a temporal code, a phase code, or a three-level phase code. In additional embodiments, the reflectors are defined by gratings.
Communication methods include selecting a set of synchronous codes and assigning a synchronous code from the set to each of a plurality of data channels. Data streams are encoded corresponding to the data channels based on respective synchronous codes. In representative embodiments, encoded data streams are combined to form a combined, coded data stream. In specific embodiments, the codes are orthogonal codes, substantially orthogonal codes, or temporal phase codes.
Additional communication methods include receiving the combined coded data stream and decoding portions of the combined coded data stream based on the synchronous codes.
Methods of code-based multiplexing in a wavelength-division multiplexed (WDM) communication system include selecting at least one carrier signal of a selected wavelength of the WDM communication system. The carrier signal is divided into portions that are modulated based on data streams and codes assigned to each portion, wherein the codes are selected from a set of substantially orthogonal, temporal codes.